Process of producing chromic-acid solutions



April 28, 1925. 1,535,100

A. -W. BURWELL PROCESS OF PRODUCING CHROMIC ACID SOLUTIONS -Filed Jan. 23, 1922 Patented Apr. 28, 1925.

UNITED STATES ARTHUR W. BURWELL, OE POUGHKEEPSIE, NEW YORK.

PROCESS OF PRODUCING CHROMIC-ACID SOLUTIONS.

Application filed January 23, 1922. Serial No. 531,281.

To all whom it may concern:

Be it known that I, ARTHUR W. BURWELL, a citizen of the United States, residing at Poughkecpsie, in the county of Dutchess and State of New York, have invented certain new and useful Improvements in Processes of Producing Chromic-Acid Solutions, of which the following is a specification.

As is well known, solutions containing available oxygen in the form of free chromic acid are commonly used for effecting oxidation of organic compounds. In such cases the oxidation generally takes place in a sulfuric acid bath, and the principal reduction product is chromic sulfate.

Many attempts have been made in the past to regenerate such exhausted solutions, usually by anodic oxidation. The present invention relates to regenerative processes of this type, the primary object of the invention being to provide a simple and efficient process whereby the electrolytic regeneration of chromium sulfate solutions may be commercially realized.

For a full understanding of the invention reference is made to the accompanying drawing wherein the figure is a central vertical section of one type of apparatus suitable for use in carrying out the invention, it being understood however that the invention is not limited to the particular arrangement or disposition of parts illustrated.

In said drawing 1 indicates a vessel which may serve both as the container for the solution and as the anode of the circuit. It may be made either of lead, or of a leadantimony alloy containing a relatively low proportion (say 35-70%) of antimony. Or, preferably, the anode may consist of a lead-antimony alloy faced upon its active surface with lead. The anode, whether of lead or lead-antimony alloy, is preferably in the form of east (as distinguished from rolled) sheets, plates or other forms; it having been found that the cast metal is decidedly more resistant to disintegration under the strongly oxidizing conditions ob taining in practice. At the same time I have observed that the highest efficiencies in the process are secured after the anode surface becomes coated with a peroxide layer; and accordingly I provide, such character of metal in the active face of the anode as will acquire readily a coating of this kind. As stated above this active lead surface is preferably of cast metal, either pure lead, or lead alloyed with antimony, which may be present in any proportion up to about seven per cent. My invention is not however limited to the use of these preferred materials.

2 indicates a porous diaphragm, illustrated as in the form of a vessel containing the cathode 3, which may also be of lead,

or lead-antimony alloy, or other metal or conductive material such as carbon which is not attacked by the solutions under the conditions of use. The diaphragm may consist of filtros, porous earthenware, or otherlir suitable porous acid-resistant material. I

4 indicates the inflow conduit for the chromium sulfate solution, and 5 the outfiow for the regenerated liquor. 6 indicates an overflow lip on the diaphragm-vessel 2. As will be noted the direction of flow of the liquor is successively through the cathode and anode compartments. 7, 7 indicate insulating supports for the porous cup. represents means for introducing air into the cathode compartment for a' purpose hereafter described.

In a typical operation in accordance with, this invention, a chromic sulfate solution, preferably of a concentration corresponding to about 10% chromic anhydrid equivalent, 18 caused to flow successively through the negative and positive compartments, overflowing from the latter. The optimum temperature is about 8595 C. The current density at the anode is preferably of the order of 18 amperes per sq. ft., that at the cathode being from five to ten times as great. The rate of flow of the solution is preferably such that the outfiowing liquor contains not to exceed 3.5% of available oxygen, corresponding to about 9% CrO As will be noted, this provides about 10% excess of chromium sulfate in the solution, which excess serves two important purposes, to wit, to insure a high efficiency of chromic acid production; and to prevent evolution. of free oxygen, which if permitted will in time cause disintegration of the anode. The inflowing solution is preferably slightly acid with sulfuric acid, but the process is operative with either neutral or basic solutions.

As illustrated the solution is delivered to the surface of the bath in both cathode and anode compartments; and this is satisfactory in practice since under the described conditions the convection currents are suffisolutions may be delivered at any point below the surface if desired.

A tendency is sometimes observed to the formation of gelatinous deposits containing chromous compounds in the-cathode compartment, or even to the deposition of chromium metal; either of which will interfere with the normal working of the process. I have found that for the best results the solution in the cathode compartment should" never become reduced below the chromic state; and have further found that this result is readily accomplished by providing a slow introduction of air, as through pipe 8, into the cathode compartment oi'- each cell.

I claim 1. Electrolytic process of oxidizing chromic salt solutions comprising causing such solutions to flow successively through the cathode and anode compartments of an electrolytic cell having a porous diaphragm and an anode presenting an active load surface; and maintaining sufficient excess of chromic salt in the solution to prevent anodic evolution of oxygen.

2. Process according to claim 1 wherein the anode has an active surface of lead alloyed with antimony.

3. Process consisting in subjecting a chromic compound'to oxidation in resence of a cast metal anode comprising lead.

4. Process accordingto claim 3 wherein the anode consists of a cast alloy of lead and antimony.

5. Electrolytic process of oxidizing chromic salt solutions comprising causing such solutions to flow successively through the cathode and anode compartments of an electrolytic cell having a porous diaphragm; and preventing cathodic reduction below the chromic state by introducing air into the cathode compartment.

6. Process consisting in subjecting a solution of a chromic compound to oxidation in an electrolytic cell in the presence of an anode consisting of an alloy of lead and antimony, and introducing air into the solution in the cathode compartment.

In testimony whereof, I aiiix my signature.

ARTHUR \V. BURWELL 

